Electrically noisy environments such as automobiles, factories, or other locations with large amounts of equipment in operation, typically use differential amplifiers to reject noise that couples onto both the positive and negative leads of an analog input. The noise, common to both the positive and negative input leads, is rejected by a differential amplifier.
FIG. 1 illustrates conventional integrated analog audio front end circuitry 100 that includes differential programmable gain amplifier (PGA) circuitry 120 which rejects common mode noise and that is coupled to receive positive and negative input signals from multiplexer circuitry 102 and to provide positive and negative output signals to analog to digital converter (ADC) 114. Multiplexer circuitry 102 is present to select among multiple analog differential audio input source pairs 1041 to 104N, each of which includes respective positive and negative signal input pads 103 and 105 that provide a respective input for positive and negative signal lines of a given analog audio source. As shown, multiplexer circuitry 102 includes input multiplexer switching elements S1 that are provided for selectively and separately coupling each individual differential source 104 one at a time to the differential programmable gain amplifier 120 as shown.
As further shown in FIG. 1, each of positive and negative input signal lines of the differential programmable gain amplifier 120 includes a respective input resistor R1 that is coupled to gain control circuit components and differential amplifier 106 of circuitry 120. Gain control circuit components include variable resistors R2 that may be selectively coupled in parallel within a respective signal line in response to a control signal provided by a microcontroller (not shown).
Any resistive mismatches between the positive and negative signal paths of analog audio front end circuitry 100 that are present in external or internal integrated circuitry components will degrade the ability of differential PGA circuitry 120 to reject common-mode noise. In the past, permanently trimmable components, such as resistors or fuses, have been provided in each of the positive and negative signal paths of such circuitry, and laser trimming or fuse trimming was used to correct for integrated resistive mismatches.